Lithium ionic conducting glass thin film and carbon dioxide sensor comprising the glass thin film

ABSTRACT

The lithium ionic conducting glass thin film of the present invention comprises a glass containing 20 to 80 mol % of an Li 2  O component. This glass thin film has a dense structure which shuts off gas and can be used as a solid electrolyte in a carbon dioxide sensor. 
     The thin carbon dioxide sensor of the present invention comprises the above lithium ionic conducting glass thin film as a solid electrolyte. This sensor enables complete elimination of noise attributed to gas permeation by virtue of the lithium ionic conducting glass thin film having a dense structure with the result that the accuracy of carbon dioxide concentration measurement can be enhanced. Further, the lithium ionic conducting glass is formed into a thin film, so that the sensor has a low internal resistance, acts at low operating temperatures and exhibits a high response speed. Therefore, a heater having a capacity smaller than in the prior art can be used in the sensor, so that, even when a structure integral with a heater and the like is employed, miniaturization of the sensor can be realized.

TECHNICAL FIELD

The present invention relates to a lithium ionic conducting glass thinfilm and a carbon dioxide sensor comprising the lithium ionic conductingglass thin film. This carbon dioxide sensor is compact, of the powersaving type and inexpensive and can measure the concentration of carbondioxide in the air.

BACKGROUND ART

The conventional carbon dioxide sensor of the solid electrolyte typecomprises a sort of battery in which a noble metal electrode and a metalcarbonate capable of creating a dissociation equilibrium with carbondioxide are arranged on one side of a bulky solid electrolyte composedof a sintered ceramic body capable of conducting alkali ions such as Na+and Li+ while a noble metal electrode is disposed on the opposite sideof the bulky solid electrolyte and in which, when carbon dioxidecontacts the metal carbonate, an electromotive force depending on theconcentration of carbon dioxide is generated between the two electrodes.This electromotive force satisfies the following Nernst formula:

    E=E.sub.0 +(RT/nF)1n(P.sub.CO2)

wherein E₀ is a constant, R is a gas constant, T is an absolutetemperature, n is the number of kinetic electrons, F is the Faradayconstant and P_(CO2) is a partial pressure of carbon dioxide. That is,the above carbon dioxide sensor enables determining the concentration ofcarbon dioxide by measuring the electromotive force.

It has been reported that, when the reference electrode of theconventional carbon dioxide sensor of the solid electrolyte type iscomposed of an oxygen ionic. conductor and a noble metal electrode, thewhole body of the carbon dioxide sensor can be inserted in a subject gasin the measurement of carbon dioxide concentration (Nobuhito IMANAKA etal, Denki Kagaku, 61, 909 (1993)).

The carbon dioxide sensor of the solid electrolyte type detects theconcentration of carbon dioxide by an electrode reaction effectedbetween the metal carbonate disposed on one side of the solidelectrolyte layer and carbon dioxide, so that the metal carbonate sideis left open to the subject atmosphere while the opposite side is leftopen to the reference concentration atmosphere (generally, the air) inthe measurement of carbon dioxide concentration. Therefore, the solidelectrolyte for use therein is required to have a dense structure whichcompletely shuts off both the atmospheres.

However, the conventionally proposed carbon dioxide sensor of the solidelectrolyte type is in the form of a pellet having a thickness rangingfrom several millimeters to several centimeters and a size ranging fromseveral millimeters square to several centimeters square and prepared bythe process such as the melt solidification process or the sinteringprocess. The sintered ceramic body having been employed as the solidelectrolyte has a porous structure, so that it occurs that carbondioxide permeates the sintered ceramic body to thereby cause sensoroutput variation, i.e., noise. This is the cause of error found incarbon dioxide concentration measurements.

The internal resistance of the carbon dioxide sensor is in directproportion to the thickness of the employed solid electrolyte. In theuse of the bulky solid electrolyte, the large thickness thereof leads toan extremely large internal resistance, so that the electromotive forceof the carbon dioxide sensor is lowered to thereby disenable accuratemeasurement of the carbon dioxide concentration. Thus, for lowering theinternal resistance, the carbon dioxide sensor must be used at hightemperatures such that the ion conductivity is high and the internalresistance is reduced, so that the operating temperature of the carbondioxide sensor is generally as very high as about 350° to 600° C. Thecarbon dioxide sensor must be provided with a heater in the vicinitythereof for achieving the practical use of the carbon dioxide sensor.The heater for use therein must have a large capacity, so that theproblem is encountered that the whole body of the carbon dioxide sensorinevitably has a large volume.

Although a thin film of solid electrolyte is especially desired forminiaturizing the whole body of the carbon dioxide sensor, obtaining athin film is difficult of a sintered body of a ceramic such asβ-alumina, NASICON (Na₁ +_(x) ZrSi_(x) P_(3-x) O₁₂ wherein x is 0-3) orLISICON (Li_(16-2x) Zn(GeO₄)₄ wherein x is 0-8) having generally beenemployed as the solid electrolyte. Even if a thin film of such asintered ceramic body is managed to obtain, the thin film formedsintered body is polycrystalline, so that there is a problem on thedenseness for completely shutting off the gas present between thedetector electrode and the reference electrode. When this thin filmformed sintered body is used as the solid electrolyte, it occurs thatthe gas permeation therethrough brings about sensor output variation tothereby result in the cause of error.

The speed of response of the carbon dioxide sensor is governed by therate of diffusion of carbon dioxide in the metal carbonate employed inthe detector electrode. The formation of the metal carbonate into a thinfilm is needed for increasing the response speed from the conventionalabout several minutes to several tens of seconds. However, the formationof the metal carbonate into a thin film is difficult in the use of theconventional melt solidification process or sintering process.

Therefore, if a lithium ionic conducting glass thin film and a thinmetal carbonate film are developed, its industrial value is striking.

An object of the present invention is to provide a lithium ionicconducting glass thin film having a dense structure which shuts offgases. Another object of the present invention is to provide a compactthin carbon dioxide sensor which is free from noise attributed to gaspermeation, exhibits a high response speed at low operating temperaturesand is of the power saving type.

DISCLOSURE OF THE INVENTION

The lithium ionic conducting (conductive) glass thin film of the presentinvention comprises a glass containing 20 to 80 mol % of an Li₂ Ocomponent.

The thin carbon dioxide sensor of the present invention comprises alithium ionic conducting (conductive) glass thin film as a solidelectrolyte.

It is preferred that the lithium ionic conducting glass thin film foruse in the thin carbon dioxide sensor be a thin film comprising amaterial represented by the formula:

    (Li.sub.2 O).sub.x --(SiO.sub.2).sub.(100-x)

wherein 20 mol %≧x mol %≧80 mol %. Especially, it is preferred that thelithium ionic conducting glass thin film be one formed by the sputteringprocess.

Preferred examples of the thin carbon dioxide sensors according to thepresent invention include:

(1) a thin carbon dioxide sensor comprises a layer of metal carbonatehaving a meshed noble metal electrode buried therein, a lithium ionicconducting glass thin film as a solid electrolyte and a layer of noblemetal electrode laminated in this sequence;

(2) a thin carbon dioxide sensor wherein a lithium ionic conductingglass thin film and a thin metal carbonate film in this sequence arelaminated to one surface of a substrate, preferably, an oxygen ionicconducting ceramic substrate and, further, a thin noble metal electrodefilm is laminated to the surface of the thin metal carbonate film, and

wherein a thin noble metal electrode film and a thin film heater arelaminated to the other surface of the substrate in a fashion such thatthe latter thin noble metal electrode film and the thin film heater arearranged so as not to contact each other;

(3) a thin carbon dioxide sensor wherein a lithium ionic conductingglass thin film and a thin noble metal electrode film in this sequenceare laminated to one surface of a substrate, preferably, an oxygen ionicconducting ceramic substrate and, further, a thin metal carbonate filmis laminated to the surface of the thin noble metal electrode film, and

wherein a thin noble metal electrode film and a thin film heater arelaminated to the other surface of the substrate in a fashion such thatthe latter thin noble metal electrode film and the thin film heater arearranged so as not to contact each other;

(4) a thin carbon dioxide sensor wherein a lithium ionic conductingglass thin film and a thin noble metal electrode film are laminated toone surface of a substrate, preferably, an oxygen ionic conductingceramic substrate in a fashion such that the lithium ionic conductingglass thin film and the thin noble metal electrode film are arranged soas not to contact each other, a thin metal carbonate film is laminatedto the surface of the lithium ionic conducting glass thin film andfurther a thin noble metal electrode film is laminated to the surface ofthe thin metal carbonate film, and

wherein a thin film heater is laminated to the other surface of thesubstrate;

(5) a thin carbon dioxide sensor wherein a lithium ionic conductingglass thin film and a thin noble metal electrode film are laminated toone surface of a substrate, preferably, an oxygen ionic conductingceramic substrate in a fashion such that the lithium ionic conductingglass thin film and the thin noble metal electrode film are arranged soas not to contact each other, a thin noble metal electrode film islaminated to the surface of the lithium ionic conducting glass thin filmand further a thin metal carbonate film is laminated to the surface ofthis thin noble metal electrode film, and

wherein a thin film heater is laminated to the other surface of thesubstrate;

(6) a thin carbon dioxide sensor wherein an oxygen ionic conductingceramic thin film is laminated to a surface of a plane heater substrate,a lithium ionic conducting glass thin film and a thin noble metalelectrode film are laminated to the surface of the oxygen ionicconducting ceramic thin film in a fashion such that the lithium ionicconducting glass thin film and the thin noble metal electrode film arearranged so as not to contact each other, a thin metal carbonate film islaminated to the surface of the lithium ionic conducting glass thin filmand further a thin noble metal electrode film is laminated to thesurface of the thin metal carbonate film; and

(7) a thin carbon dioxide sensor wherein an oxygen ionic conductingceramic thin film is laminated to a surface of a plane heater substrate,a lithium ionic conducting glass thin film and a thin noble metalelectrode film are laminated to the surface of the oxygen ionicconducting ceramic thin film in a fashion such that the lithium ionicconducting glass thin film and the thin noble metal electrode film arearranged so as not to contact each other, a thin noble metal electrodefilm is laminated to the surface of the lithium ionic conducting glassthin film and further a thin metal carbonate film is laminated to thesurface of this thin noble metal electrode film.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1, 2, 3 and 4 are explanatory diagrams showing examples of thestructures of the thin carbon dioxide sensors according to the presentinvention.

FIG. 5 is a graph showing the relationship between the electromotiveforce and the concentration of carbon dioxide at an operatingtemperature of 400° C. as measured by the use of the thin carbon dioxidesensor of Example 1 of the present invention.

FIG. 6 is a view showing electromotive force response waveforms of thethin carbon dioxide sensor of Example 2 of the present invention tovarious carbon dioxide concentrations.

FIG. 7 is a graph showing the relationship between the electromotiveforce and the concentration of carbon dioxide at an operatingtemperature of 400° C. as measured by the use of the thin carbon dioxidesensor of Example 2 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The lithium ionic conducting glass thin film of the present inventionand the thin carbon dioxide sensor comprising the glass thin film willnow be described in detail.

Lithium ionic conducting class thin film

The lithium ionic conducting glass thin film of the present inventioncomprises a glass containing 20 to 80 mol % of an Li₂ O component and isa solid electrolyte.

Examples of the lithium ionic conducting glass thin films include glassthin films comprising respective materials represented by the formulae:

    (Li.sub.2 O).sub.x --(SiO.sub.2).sub.(100-x)

wherein 20 mol %≧x mol %≧80 mol %;

    (Li.sub.2 O).sub.x --(P.sub.2 O.sub.5).sub.(100-x)

wherein 30 mol %≧x mol %≧70 mol %;

    (Li.sub.2 O).sub.x --(B.sub.2 O.sub.3).sub.(100-x)

wherein 30 mol %≧x mol %≧70 mol %; and

    (Li.sub.2 O).sub.x --(Al.sub.2 O.sub.3).sub.(100-x)

wherein 30 mol %≧x mol %≧70 mol %.

The above lithium ionic conducting glass thin film can generally beformed by, for example, any of the sputtering, ion plating, ion beamevaporation, CVD, vacuum evaporation, electron beam evaporation, screenprinting, spin coating and sol gel processes. Of these, the lithiumionic conducting glass thin film formed by the sputtering process ispreferred.

The lithium ionic conducting glass thin film of the present invention isa glass formed by the film forming process such as the sputteringprocess, so that it has a dense structure and is excellent in capabilityof shutting off gases. In this connection, Nobuhito IMANAKA et al, DenkiKagaku, 61, 909 (1993) describe a sintered body, as a lithium ionicconductor, prepared from a powder obtained by mixing together Li₂ CO₃,TiO₂, (NH₄)H₂ PO₄ and Li3PO4 in a molar ratio of 0.5:2:3:0.2.

Thin carbon dioxide sensor

The thin carbon dioxide sensor of the present invention will bedescribed below with reference to examples of thin carbon dioxidesensors illustrated in FIGS. 1 to 4.

First, the thin carbon dioxide sensor illustrated in FIG. 1 will bedescribed.

The thin carbon dioxide sensor of FIG. 1 includes a layer of metalcarbonate 1 having a meshed noble metal electrode 2 buried therein, alithium ionic conducting glass thin film 3 as a solid electrolyte and alayer of noble metal electrode 4 laminated in this sequence. The noblemetal electrodes 2 and 4 are provided with respective leads 5 and 6 forelectromotive force measurement.

In this arrangement, the metal carbonate 1 side is brought into contactwith the measured atmosphere while the noble metal electrode 4 side isbrought into contact with the reference atmosphere such as the air. Thetwo atmospheres are partitioned from each other.

Examples of the above metal carbonates 1 include lithium carbonate,sodium carbonate, potassium carbonate, barium carbonate, strontiumcarbonate and calcium carbonate. Of these, lithium carbonate or amixture of lithium carbonate and another carbonate, especially, bariumcarbonate or calcium carbonate is preferably used.

The thickness of the layer of the above metal carbonate depends on thetype of the metal carbonate but generally ranges from 0.01 to 3 mm andpreferably from 0.05 to 1 mm.

The lithium ionic conducting glass thin film 3 for use in this inventionis the above lithium ionic conducting glass thin film of the presentinvention and has a dense structure.

Examples of the lithium ionic conducting glass thin films are as setforth above. Of them, preferred examples are glass thin films composedof respective materials represented by the formula:

    (Li.sub.2 O).sub.x --(SiO.sub.2).sub.(100-x)

wherein 20 mol %≧x mol %≧80 mol %. Especially preferred examples areglass thin films composed of respective materials represented by theabove formula in which x ranges from 40 to 60.

For example, Nb₂ O₅, Ta₂ O₅ or WO₃ may be added to (Li₂ O)_(x)--(SiO₂).sub.(100-x) in an amount of preferably 1 to 20 mol % and morepreferably 5 to 15 mol % so as to increase the crystallizationtemperature and the ionic conductivity.

In particular, the lithium ionic conducting glass thin film formed bythe sputtering process is preferred.

In the present invention, preferred use is made of a glass thin film of(Li₂ O)₄₅ --(SiO₂)₅₅ formed by the sputtering process.

The use of the above lithium ionic conducting glass thin film as thesolid electrolyte 3 enables miniaturization of the sensor, so thatminiaturization of the whole body of the carbon dioxide sensor togetherwith power saving can be attained by the employment of a heater with acapacity smaller than in the prior art.

The thickness of the above lithium ionic conducting glass thin filmgenerally ranges from 0.5 to 10 μm and preferably from 1 to 5 μm.

Each of the above noble metal electrodes 2 and 4 may be composed of, forexample, any of platinum, gold and silver, of which platinum and goldare preferred.

Each of the above leads 5 and 6 may be composed of, for example, any ofplatinum, gold and silver, of which platinum and gold are preferred.

Now, with respect to the carbon dioxide sensors of FIGS. 2 to 4, thestructures thereof will first be described and the description of theconstituent materials will follow thereafter.

Structure of thin carbon dioxide sensor of FIG. 2

In the thin carbon dioxide sensor of FIG. 2, a lithium ionic conductingglass thin film 8 and a thin metal carbonate film 9 in this sequence arelaminated to one surface of an oxygen ionic conducting ceramic substrate7, preferably, in the entirety of the surface and, further, a thin noblemetal electrode film 10 is laminated to part of the surface of the thinmetal carbonate film 9.

Furthermore, a thin noble metal electrode film 11 and a thin film heater12 are laminated to the other surface of the oxygen ionic conductingceramic substrate 7 in a fashion such that the thin noble metalelectrode film 11 and the thin film heater 12 are arranged so as not tocontact each other.

The thin noble metal electrode films 10 and 11 are provided withrespective leads 13 and 14 for electromotive force measurement. The thinfilm heater 12 is provided with a heater lead 15.

The thin noble metal electrode film 10 and the thin metal carbonate film9 structures a detecting electrode. The oxygen ionic conducting ceramicsubstrate 7 and the thin noble metal electrode film 11 structures areference electrode. The thin noble metal electrode film 10 and the thinmetal carbonate film 9 structuring a detecting electrode may belaminated in reverse sequence.

Structure of thin carbon dioxide sensor of FIG. 3

In the thin carbon dioxide sensor of FIG. 3, a lithium ionic conductingglass thin film 17 and a thin noble metal electrode film 18 arelaminated to one surface of an oxygen ionic conducting ceramic substrate16 in a fashion such that the lithium ionic conducting glass thin film17 and the thin noble metal electrode film 18 are arranged so as not tocontact each other, a thin metal carbonate film 19 is laminated to theentire surface of the lithium ionic conducting glass thin film 17 andfurther a thin noble metal electrode film 20 is laminated to part of thesurface of the thin metal carbonate film 19.

Further, a thin film heater 21 is laminated to the other surface of theoxygen ionic conducting ceramic substrate 16.

The thin noble metal electrode films 18 and 20 are provided withrespective leads 22 and 23 for electromotive force measurement. The thinfilm heater 21 is provided with a heater lead 24.

The thin noble metal electrode film 20 and the thin metal carbonate film19 structures a detector electrode. The oxygen ionic conducting ceramicsubstrate 16 and the thin noble metal electrode film 18 structures areference electrode. The thin noble metal electrode film 20 and the thinmetal carbonate film 19 structuring a detecting electrode may belaminated in reverse sequence.

Structure of thin carbon dioxide sensor of FIG. 4

In the thin carbon dioxide sensor of FIG. 4, an oxygen ionic conductingceramic thin film 26 is laminated to a surface, preferably, the entiresurface of a plane heater substrate 25, a lithium ionic conducting glassthin film 27 and a thin noble metal electrode film 28 are laminated tothe surface of the oxygen ionic conducting ceramic thin film 26 in afashion such that the lithium ionic conducting glass thin film 27 andthe thin noble metal electrode film 28 are arranged so as not to contacteach other, a thin metal carbonate film 29 is laminated to the entiresurface of the lithium ionic conducting glass thin film 27 and further athin noble metal electrode film 30 is laminated to the surface of thethin metal carbonate film 29.

The thin noble metal electrode films 28 and 30 are provided withrespective leads 31 and 32 for electromotive force measurement. The thinfilm heater 25 is provided with a heater lead 33.

The thin noble metal electrode film 30 and the thin metal carbonate film29 structures a detecting electrode. The oxygen ionic conducting ceramicsubstrate 26 and the thin noble metal electrode film 28 structures areference electrode. The thin noble metal electrode film 30 and the thinmetal carbonate film 29 structuring a detecting electrode may belaminated in reverse sequence.

Components of thin carbon dioxide sensors of FIGS. 2 to 4

Examples of the oxygen ionic conducting ceramic substrates 7 and 16 ofFIGS. 2 and 3 include substrates of zirconia stabilized with any of theoxides Y₂ O₃, MgO and CaO. The content of each of the oxides Y₂ O₃, MgOand CaO in the substrate ranges from 5 to 20 mol %.

The thickness of each of the above substrates preferably ranges from 20μm to 1 mm.

The lithium ionic conducting glass thin films 8, 17 and 27 of FIGS. 2 to4 are identical with the above lithium ionic conducting glass thin film3 of FIG. 1.

The thickness of each of the lithium ionic conducting glass thin filmspreferably ranges from 0.1 to 20 μm and more preferably from 0.5 to 5μm.

The respective metal carbonates composing the thin metal carbonate films9, 19 and 29 of FIGS. 2 to 4 are identical with the metal carbonateemployed in the layer of metal carbonate 1 of FIG. 1. It is preferablylithium carbonate or a mixture of lithium carbonate and anothercarbonate.

The above thin metal carbonate film can generally be formed by, forexample, any of the sputtering, ion plating, ion beam evaporation, CVD,vacuum beam evaporation, electron beam evaporation, screen printing,spin coating and sol gel processes. Of these, the thin metal carbonatefilm formed by the sputtering process is preferred.

The thickness of the thin metal carbonate film preferably ranges from0.1 to 20 μm and more preferably from 0.5 to 5 μm.

A preferred example of the thin noble metal electrode films 10, 11, 18,20, 28 and 30 of FIGS. 2 to 4 is a thin film of gold or platinum.

The above thin noble metal electrode film can generally be formed by,for example, any of the sputtering, ion plating, ion beam evaporation,CVD, vacuum evaporation, electron beam evaporation, screen printing,spin coating and sol gel processes. Of these, the thin noble metalelectrode film formed by the sputtering process is preferred. In thisprocess, a porous thin noble metal electrode film of high gas permeationcan be formed by controlling sputtering conditions. This thin noblemetal electrode film can be used in combination with a meshed noblemetal electrode.

The thickness of the thin noble metal electrode film preferably rangesfrom 0.1 to 10 μm and more preferably from 0.2 to 2 μm.

Examples of the oxygen ionic conducting ceramic thin film 26 of FIG. 4include substrates of zirconia stabilized with any of the oxides Y₂ O₃,MgO and CaO. The content of each of the oxides Y₂ O₃, MgO and CaO in thesubstrate ranges from 5 to 20 mol %.

The above oxygen ionic conducting ceramic thin film can generally beformed by, for example, any of the sputtering, ion plating, ion beamevaporation, CVD, vacuum evaporation, electron beam evaporation, screenprinting, spin coating and sol gel processes. Of these, the oxygen ionicconducting ceramic thin film formed by the sputtering process ispreferred.

The thickness of the oxygen ionic conducting ceramic thin filmpreferably ranges from 0.1 to 20 μm and still preferably from 0.5 to 5μm.

Each of the thin film heaters 12 and 21 of FIGS. 2 and 3 is composed ofa platinum/rhodium alloy, a platinum/palladium alloy, ruthenium oxide orthe like.

The above thin film heater can generally be formed by, for example, anyof the sputtering, ion plating, ion beam evaporation, CVD, vacuumevaporation, electron beam evaporation, screen printing, spin coatingand sol gel processes. Of these, the thin film heater formed by thesputtering process is preferred.

The thickness of the thin film heater preferably ranges from 0.1 to 10μm and more preferably from 0.2 to 2 μm. The plane heater substrate 25of FIG. 4 is composed of a platinum/rhodium alloy, a platinum/palladiumalloy, ruthenium oxide or the like.

The thickness of the plane heater substrate preferably ranges from 20 μmto 1 mm.

Effect of the Invention

The lithium ionic conducting glass thin film of the present inventioncomprises a glass containing 20 to 80 mol % of an Li₂ O component, sothat it has a dense structure which shuts off gases and can be used as asolid electrolyte of a carbon dioxide sensor.

Further, the lithium ionic conducting glass thin film of the presentinvention finds applications in a thin lithium secondary-battery(microbattery), an analog memory, an electrochromic element (dimmerglass) and the like.

The solid electrolyte of the thin carbon dioxide sensor of the presentinvention comprises a lithium ionic conducting glass thin film having adense structure, so that, even if the carbon dioxide sensor is used inthe form of a thin film, it enables complete elimination of noiseattributed to gas permeation,

thereby realizing an improvement of accuracy in measurement of carbondioxide concentration as well as miniaturization of the sensor.

Further, thin carbon dioxide sensor of the present invention is formedinto a thin film, so that the sensor has a low internal resistance, actsat low operating temperatures and exhibits a high response speed.Therefore, a heater having a capacity smaller than in the prior art canbe used in the thin carbon dioxide sensor of the present invention, sothat, even when a structure integral with a heater and the like isemployed, miniaturization and power saving can be attained for theintegrated carbon dioxide sensor.

Moreover, the whole body of the thin carbon dioxide sensor of thepresent invention in which an oxide ionic conducting ceramic is used asa substrate or thin film can be inserted in a subject gas in themeasurement of carbon dioxide concentration because the oxygen ionicconducting ceramic shuts off ions other than oxygen ions.

The thin carbon dioxide sensor of the present invention can findapplications in various fields such as environment, agriculture andmedical care.

EXAMPLES

The present invention will now be illustrated with reference to thefollowing Examples, which in no way limit the scope of the invention.

Example 1

Production of thin carbon dioxide sensor of structure shown in FIG. 1

A reagent of lithium carbonate was weighed in a predetermined amount,put in an alumina crucible and heated in an electric furnace at about750° C. for about 3 hr. A meshed platinum (100 mesh) as an electrode 2was dipped in the thus molten lithium carbonate to effect coating. Thus,a layer of metal carbonate 1 having a thickness of about 0.5 mm wasformed.

Subsequently, oxygen-reactive sputtering was performed on one surface ofthe layer of metal carbonate 1 with the use of a sintered body of Li₂SiO₃ as a target, thereby forming a solid electrolyte of an (Li₂ O)₅₀--(SiO₂)₅₀ film 3 having a thickness of about 1 μm.

Then, a platinum sputter film of about 0.5 μm in thickness was formed asan electrode 4 on the solid electrolyte.

Thereafter, platinum leads 5 and 6 were connected to the electrode 2composed of platinum mesh and the electrode 4 composed of platinumsputter film, respectively. Thus, a carbon dioxide sensor was obtained.

The side of metal carbonate 1 of the thus obtained sensor was exposed toa subject atmosphere whose carbon dioxide concentration changed from 500ppm to 1% by volume while the platinum sputter film side of the sensorwas exposed to the air, and the electromotive force generated betweenthe platinum leads 5 and 6 was measured at an operating temperature of400° C. During the measurement, the subject atmosphere and the air asthe reference atmosphere was partitioned by means of a glass tube.

The measurement results are shown in FIG. 5.

FIG. 5 demonstrates that a thin carbon dioxide sensor exhibiting a goodlinearity in accordance with the Nernst formula was obtained in Example1.

Example 2

Production of thin carbon dioxide sensor of structure shown in FIG. 3

A lithium ionic conducting glass thin film 17 of about 1 μm in thicknesswas formed as a solid electrolyte on part of a surface of a ZrO₂substrate 16 (size: 10 mm×10 mm×0.5 mm) having a Y₂ O₃ content of 8 mol%. The lithium ionic conducting glass thin film 17 was prepared by theoxygen-reactive radio-frequency (RF) magnetron sputtering process withthe use of molten lithium silicate (Li₂ SiO₃) as a target, and itscomposition was regulated to (Li₂ O)₄₅ --(SiO₂)₅₅ by a quartz chipplaced on the target. The crystallization temperature of the lithiumionic conducting glass thin film 17 is not lower than 600° C.

Subsequently, a thin lithium carbonate film of about 1 μm in thicknessas a thin metal carbonate film 19 constituting a detector electrode anda thin gold film of about 0.21 μm in thickness as a thin noble metalelectrode film 20 were laminated to the above lithium ionic conductingglass thin film 17. The thin lithium carbonate film was prepared by theradio-frequency magnetron sputtering process with the use of moltenlithium carbonate as a target. The thin gold film was prepared by thedirect-current (DC) magnetron sputtering process with the use of gold asa target.

Thereafter, a thin gold film as a thin noble metal electrode film 18constituting a reference electrode was formed on the same side of thesubstrate 16 as provided with the laminate of the lithium ionicconducting glass thin film 17, the thin metal carbonate film 19 and thethin noble metal electrode film 20 with a spacing given between thisthin gold film and the laminate. This thin gold film was prepared in thesame manner as employed to obtain the above thin gold film as the thinnoble metal electrode film 20.

A thin film heater 21 was formed on the back of the oxygen ionicconducting ceramic substrate 16, i.e., the surface of the oxygen ionicconducting ceramic substrate 16 on which the above laminate of thelithium ionic conducting glass thin film 17, the thin noble metalelectrode film 18, etc. was not provided. The thin film heater 21 was athin platinum film of 2 μm in thickness and was prepared by thedirect-current (DC) magnetron sputtering process with the use ofplatinum as a target.

Finally, gold leads as leads 22 and 23 for electromotive forcemeasurement were connected by ultrasonic fusion to the thin gold film asthe thin noble metal electrode film 18 and the thin gold film as thethin noble metal electrode film 20, respectively. Thus, a carbon dioxidesensor was obtained.

The whole body of the thus obtained sensor was inserted in subjectatmospheres of varied carbon dioxide concentrations, and theelectromotive force generated between the gold lead as lead 22 forelectromotive force measurement and the gold lead 23 as lead forelectromotive force measurement was measured at an operating temperatureof 400° C. Direct-current voltage was applied to the thin film heater 21so as to hold the operating temperature constant at 400° C.

Measured electromotive force response waveforms to various carbondioxide concentrations are shown in FIG. 6. The 90% response speed ofelectromotive force was about 10 sec at the first transition (194 ppm9840 ppm) and about 30 sec at the last transition (9840 ppm 194 ppm).This shows a striking improvement of the 90% response speed which hasinevitably been about several minutes.

Further, the relationship between measured carbon dioxide concentrationand electromotive force is shown in FIG. 7. As apparent from FIG. 7, athin carbon dioxide sensor exhibiting a good linearity in accordancewith the Nernst formula was obtained in Example 2.

The same results as obtained with the thin carbon dioxide sensor ofExample 2 are exhibited by the thin carbon dioxide sensors havingstructures shown in FIGS. 2 and 4 as well.

I claim:
 1. A thin carbon dioxide sensor comprising a lithium ionicconducting glass thin film solid electrolyte with 0.1 to 20 μm thicknessconsisting essentially of a glass containing 20 to 80 mol % of a Li₂ Ocomponent and 80 to 20 mol % of a member selected from the groupconsisting of SiO₂, P₂ O₅, B₂ O₃ and Al₂ O₃ and an electrode comprisinga carbonate material for detecting carbon dioxide.
 2. The thin carbondioxide sensor as claimed in claim 1, wherein the lithium ionicconducting glass thin film solid electrolyte consists essentially of amaterial represented by the formula:

    (Li.sub.2 O).sub.x --(SiO.sub.2).sub.(100-x)

wherein 20 mol %≧x mol %≧80 mol %.
 3. The thin carbon dioxide sensor asclaimed in claim 2, wherein the lithium ionic conducting glass thin filmis formed by the sputtering process.
 4. The thin carbon dioxide sensoras claimed in claim 1, wherein the lithium ionic conducting glass thinfilm and a thin metal carbonate film in this sequence are laminated toone surface of an oxygen ionic conducting ceramic substrate and furthera first thin noble metal electrode film is laminated to the surface ofthe thin metal carbonate film, andwherein a second thin noble metalelectrode film and a thin film heater are laminated to the other side ofsaid surface of the oxygen ionic conducting ceramic substrate in afashion such that the second thin noble metal electrode film and thethin film heater are arranged so as not to contact each other.
 5. Thethin carbon dioxide sensor as claimed in claim 1, wherein the lithiumionic conducting glass thin film and a first thin noble metal electrodefilm in this sequence are laminated to one surface of an oxygen ionicconducting ceramic substrate and further a thin metal carbonate film islaminated to the surface of the first thin noble metal electrode film,andwherein a second thin noble metal electrode film and a thin filmheater are laminated to the other side of said surface of the oxygenionic conducting ceramic substrate in a fashion such that the secondthin noble metal electrode film and the thin film heater are arranged soas not to contact each other.
 6. The thin carbon dioxide sensor asclaimed in claim 1, wherein the lithium ionic conducting glass thin filmand a first thin noble metal electrode film are laminated to one surfaceof an oxygen ionic conducting ceramic substrate in a fashion such thatthe lithium ionic conducting glass thin film and the first thin noblemetal electrode film are arranged so as not to contact each other, athin metal carbonate film is laminated to the surface of the lithiumionic conducting glass thin film and further a second thin noble metalelectrode film is laminated to the surface of the thin metal carbonatefilm, andwherein a thin film heater is laminated to the other side ofsaid surface of the oxygen ionic conducting ceramic substrate.
 7. Thethin carbon dioxide sensor as claimed in claim 1, wherein the lithiumionic conducting glass thin film and a first thin noble metal electrodefilm are laminated to one surface of an oxygen ionic conducting ceramicsubstrate in a fashion such that the lithium ionic conducting glass thinfilm and the first thin noble metal electrode film are arranged so asnot to contact each other, a second thin noble metal electrode film islaminated to the surface of the lithium ionic conducting glass thin filmand further a thin metal carbonate film is laminated to the surface ofthe second thin noble metal electrode film, andwherein a thin filmheater is laminated to the other side of said surface of the oxygenionic conducting ceramic substrate.
 8. The thin carbon dioxide sensor asclaimed in claim 1, wherein an oxygen ionic conducting ceramic thin filmis laminated to a surface of a plane heater substrate, the lithium ionicconducting glass thin film and a first thin noble metal electrode filmare laminated to the surface of the oxygen ionic conducting ceramic thinfilm in a fashion such that the lithium ionic conducting glass thin filmand the first thin noble metal electrode film are arranged so as not tocontact each other, a thin metal carbonate film is laminated to thesurface of the lithium ionic conducting glass thin film and further asecond thin noble metal electrode film is laminated to the surface ofthe thin metal carbonate film.
 9. The thin carbon dioxide sensor asclaimed in claim 1, wherein an oxygen ionic conducting ceramic thin filmis laminated to a surface of a plane heater substrate, the lithium ionicconducting glass thin film and a first thin noble metal electrode filmare laminated to the surface of the oxygen ionic conducting ceramic thinfilm in a fashion such that the lithium ionic conducting glass thin filmand the first thin noble metal electrode film are arranged so as not tocontact each other, a second thin noble metal electrode film islaminated to the surface of the lithium ionic conducing glass thin filmand further a thin metal carbonate film is laminated to the surface ofthe second thin noble metal electrode film.
 10. The thin carbon dioxidesensor as claimed in claim 1, wherein an oxygen ionic conducting ceramicsubstrate is laminated to a surface of a plane heater substrate, thelithium ionic conducting glass thin film and a first thin noble metalelectrode film are laminated to the surface of the oxygen ionicconducting ceramic substrate in a fashion such that the lithium ionicconducting glass thin film and the first thin noble metal electrode filmare arranged so as not to contact each other, a thin metal carbonatefilm is laminated to the surface of the lithium ionic conducting glassthin film and further a second thin noble metal electrode is laminatedto the surface of the thin metal carbonate film.
 11. The thin carbondioxide sensor as claimed in claim 1, wherein an oxygen ionic conductingceramic substrate is laminated to a surface of a plane heater substrate,the lithium ionic conducting glass thin film and a first thin noblemetal electrode film are laminated to the surface of the oxygen ionicconducting ceramic substrate in a fashion such that the lithium ionicconducting glass thin film and the first thin noble metal electrode filmare arranged so as not to contact each other, a second thin noble metalelectrode film is laminated to the surface of the lithium ionicconducting glass thin film and further a thin metal carbonate film islaminated to the surface of the second thin metal electrode film. 12.The thin carbon dioxide sensor as claimed in claim 1, wherein thelithium ionic conducting glass thin film is formed on an oxygen ionicconducting ceramic substrate.
 13. A thin carbon dioxide sensorcomprising a lithium ionic conducting glass thin film solid electrolyte0.1 to 20 μm thick and consisting essentially of a glass containing 20to 80 mol % of an LiO₂ component and 80 to 20 mol % of a SiO₂ component,a detecting electrode comprising a first thin noble metal electrode filmand a thin metal carbonate film and a reference electrode comprising anoxygen ionic conducting ceramic substrate and a second thin noble metalelectrode film, wherein the lithium ionic conducting glass thin filmsolid electrolyte is disposed between the detecting electrode and thereference electrode.
 14. The thin carbon dioxide sensor as claimed inclaim 13, wherein the lithium ionic conducting glass thin film contains40 to 60 mol % of a LiO₂ component and 60 to 40 mol % of a SiO₂component.
 15. The thin carbon dioxide sensor as claimed in claim 13,wherein the lithium ionic conducting glass thin film is formed by thesputtering process.
 16. The thin carbon dioxide sensor as claimed inclaim 13, wherein the lithium ionic conducting glass thin film and athin metal carbonate film with 0.1 to 20 μm thickness in this sequenceare laminated to one surface of an oxygen ionic conducting ceramicsubstrate with 20 μm to 1 mm thickness and further the first thin noblemetal electrode film is laminated to the surface of the thin metalcarbonate film, andwherein the second thin noble metal electrode filmand a thin film heater are laminated to the other side of said surfaceof the oxygen ionic conducting ceramic substrate in a fashion such thatthe second thin noble metal electrode film and the thin film heater arearranged so as not to contact each other.
 17. The thin carbon dioxidesensor as claimed in claim 13, wherein the lithium ionic conductingglass thin film and the first thin noble metal electrode film in thissequence are laminated to one surface of an oxygen ionic conductingceramic substrate with 20 μm to 1 mm thickness and further a thin metalcarbonate film with 0.1 to 20 μm thickness is laminated to the surfaceof the first thin noble metal electrode film, andwherein the second thinnoble metal electrode film and a thin film heater are laminated to theother side of the surface of the oxygen ionic conducting ceramicsubstrate in a fashion such that the second thin noble metal electrodefilm and the thin film heater are arranged so as not to contact eachother.
 18. The thin carbon dioxide sensor as claimed in claim 13,wherein the lithium ionic conducting glass thin film and the second thinnoble metal electrode film are laminated to one surface of an oxygenionic conducting ceramic substrate with 20 μm to 1 mm thickness in afashion such that the lithium ionic conducting glass thin film and thethin noble metal electrode film are arranged so as not to contact eachother, a thin metal carbonate film with 0.1 to 20 μm thickness islaminated to the surface of the lithium ionic conducting glass thin filmand further the first thin noble metal electrode film is laminated tothe surface of the thin metal carbonate film, andwherein a thin filmheater is laminated to the other side of said surface of the oxygenionic conducting ceramic substrate.
 19. The thin carbon dioxide sensoras claimed in claim 13, wherein the lithium ionic conducting glass thinfilm and the second thin noble metal electrode film are laminated to onesurface of an oxygen ionic conducting ceramic substrate with 20 μm to 1mm thickness in a fashion such that the lithium ionic conducting glassthin film and the second thin noble metal electrode film are arranged soas not to contact each other, the first thin noble metal electrode filmis laminated to the surface of the lithium ionic conducting glass thinfilm and further a thin metal carbonate film with 0.1 to 20 μm thicknessis laminated to the surface of the first thin noble metal electrodefilm, andwherein a thin film heater is laminated to the other side ofsaid surface of the oxygen ionic conducting ceramic substrate.
 20. Thethin carbon dioxide sensor as claimed in claim 13, wherein an oxygenionic conducting ceramic thin film with 0.1 to 20 μm thickness islaminated to a surface of a plane heater substrate, the lithium ionicconducting glass thin film and the second thin noble metal electrodefilm are laminated to the surface of the oxygen ionic conducting ceramicthin film in a fashion such that the lithium ionic conducting glass thinfilm and the second thin noble metal electrode film are arranged so asnot to contact each other, a thin metal carbonate film with 0.1 to 20 μmthickness is laminated to the surface of the lithium ionic conductingglass thin film and further the first thin noble metal electrode film islaminated to the surface of the thin metal carbonate film.
 21. The thincarbon dioxide sensor as claimed in claim 13, wherein an oxygen ionicconducting ceramic thin film with 0.1 to 20 μm thickness is laminated toa surface of a plane heater substrate, the lithium ionic conductingglass thin film and the second thin noble metal electrode film arelaminated to the surface of the oxygen ionic conducting ceramic thinfilm in a fashion such that the lithium ionic conducting glass thin filmand the second thin noble metal electrode film are arranged so as not tocontact each other, the first thin noble metal electrode film islaminated to the surface of the lithium ionic conducting glass thin filmand further a thin metal carbonate film with 0.1 to 20 μm thickness islaminated to the surface of the first thin noble metal electrode film.